When driving a vehicle fitted with a change gear ratio transmission connected to an engine by a torque converter the operation of the transmission is characterised by the slip and torque multiplication available from the torque converter section. The “feel” of the transmission is recognisable by an “urge to move” when the vehicle is stationary. This urge to move is caused by the stall torque of the torque converter at engine idle speed. On an uphill incline it can prevent the vehicle moving backward when the vehicle is stationary and the driver moves his foot from the brake to the accelerator pedal.
An Automated Mechanical Transmission (AMT) includes a change gear transmission and normally also includes a Central Processing Unit (CPU) which controls the selection of gear and the operation of the gear selector mechanism. Normally, an AMT is coupled to a conventional friction clutch, rather than a known hydraulic torque converter. In more advanced versions of the AMT, the clutch control is also automated and the driver has only two pedals in the cab, a throttle control or accelerator pedal and a brake pedal. Normal or manually operated clutches control the clutch and do not present the driver with an “urge to move” feeling. When stationary, the clutch is normally completely disengaged in order to minimise the clutch wear and avoid excessive heat dissipation that would arise from a continually slipping clutch. In these conditions there is nothing to prevent a vehicle fitted with an AMT rolling forwards or backwards if the driver has not applied either the parking brake or service brake using the pedal.
In a commercial vehicle having 6, 9 or even more forward speed ratios there is the possibility of selecting a gear other than first gear for engagement. Commonly in such vehicles with a 6 speed transmission the starting gear will be 2nd or 3rd. For heavier vehicles with 12 or 16 speed transmissions the starting gear will be 3rd or 5th and occasionally 7th if the vehicle is unladen and setting off in a downhill direction. With the choice of a number of start ratios it is possible to select a ratio which can minimise the clutch slip and heat generation during the launch phase.
It is desirable to present the driver with an indication the vehicle is ready for launch and can move off quickly when commanded to do so. In this specification the expression “urge to move” is intended to mean that a vehicle is ready to move and the drive line slack has been taken up and there is a sensation given to the driver that the vehicle will move off if the brakes are released. It does not necessarily mean that the vehicle will creep forward (or backwards), as would tend to be the case, for a vehicle fitted with a conventional torque converter, but it is intended to indicate that sufficient torque is being transmitted through the vehicle drive-line to take up any slack in the drive-line components and that the vehicle will not just roll in a downhill direction if it is at rest on a gradient without a brake applied. The Urge Torque is the amount of torque required to give the urge to move feeling and make the vehicle feel as if it is ready to move off from rest. It is an empirically determined figure that will depend on the weight and type of vehicle. It can be qualitatively described as the amount of torque needed to provide the driver with the sensation that at least any slack in the drive-line has been partially taken up and the clutch engagement process has started and so the vehicle is ready to move off.
Allowing the partial engagement of the clutch to provide slip under zero speed conditions partially replicates the sensations of a torque converter type system and provides the urge to move feeling experienced with such systems. An automated clutch system can be programmed to transmit a greater amount of torque and so provide a limited amount of movement and so replicate the “creep” function. Alternatively, when a vehicle is on an uphill gradient it is desirable to provide a hill holding capability in which the vehicle can be held stationary by the engine running at or slightly above idle speed and slipping clutch without the need for application of an additional braking force. It is also clear that when a vehicle is laden the amount of torque required to prevent the vehicle rolling backwards is greater than that required when it is unladen or on a flat or level surface. Clearly, it is desirable to ensure enough torque is available to provide the urge to move in the particular circumstances which the vehicle minimise the amount of slip and therefore heat generated in order to maximise clutch life.